In various plants including a nuclear power plant, a pipe is installed to allow fluid, for example, gas such as air, nitrogen, or vapor, or liquid such as water to flow therethrough. The pipe is sometimes provided with a valve device that opens and closes a flow passage of the fluid to adjust the flow rate of the fluid flowing through the flow passage. As the valve device, a butterfly valve is known which includes a valve box having a flow passage through which fluid flows, a plate-shaped valve body that opens and closes the flow passage, and a rotatable valve shaft that engages with the valve body to pivot the valve body.
As the above-described butterfly valve, a so-called non-sliding butterfly valve is known which is configured so that an outer peripheral portion of a valve body and a valve box do not slide when the valve body pivots.
For example, PTL 1 describes a butterfly valve including a motion converting mechanism that converts the rotary motion of a rotation shaft serving as a valve shaft into a nearly linear motion of a valve body in response to the rotation of the rotation shaft at a predetermined angle position in order to separate the valve body engaged with a valve seat surface of a valve box from the valve seat surface and to engage the valve body separated from the valve seat surface of the valve box with the valve seat surface of the valve box.